It is known that porous silicon Si and alumina Al2O3 substrates are useful as photonic crystals. Techniques for fabricating these porous substrates are known and have for instance been described by Ottow et al in Appl. Phys. A63, 153-159 (1996) and Schilling et al in Appl. Phys. Lett. Vol. 78, No. 9, 26 Feb. 2001, 1180-1182.
It would be desirable to be able to line or completely fill the pores in such substrates with a material of high refractive index n, n being variable with voltage. A variety of ferroelectric materials have this property, as do some non-ferroelectric materials. This would enable production of voltage-tunable photonic devices.
Zhou et al in Appl. Phys. Lett., Vol. 78, No. 5, 29 Jan. 2001, 661-663, describe the production of an SiO2 colloid crystal infilled with ferroelectric BaTiO3, which enhances the photonic band gap. However, this structure does not permit attachment of electrodes and does not permit voltage tunability.
Waser et al in Integrated Ferroelectrics, 2001, vol. 36, pp 3-20, review the known chemical deposition techniques. Waser et al primarily discuss thin film deposition but do describe attempts to fill holes in pore-patterned Al2O3 films with lead zirconate titanate PZT using chemical solution deposition (CSD). However, they report problems in filling the pores completely.
Related problems arise with silicon integrated circuits, in particular random access memories (RAMs). There is a need to deposit uniform coatings on the inside walls of trenches for silicon Si random-access memories, both dynamic DRAMs and non-volatile ferroelectric FRAMs. These trenches have high aspect ratio (ratio of depth to diameter), for instance 60:1 at 6-micron depth.
Dielectric materials for RAMs are generally deposited via chemical vapour deposition (CVD), sputtering, or related techniques in which the principal aims are to achieve uniform thickness along the inside walls. However, presently known systems have not solved the problem of achieving uniform deposition along substantially the entire surface of the trench.